In a letter to the U.S. District judge who presided over his trial, Rezko, who was convicted this month of 16 corruption-related counts, including fraud and money laundering, called prosecutors “overzealous.” And he singled out what he said were their efforts to get him to turn on Obama, an Illinois senator and the presumptive Democratic presidential nominee, and Illinois Gov. Rod Blagojevich.

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“They are pressuring me to tell them the ‘wrong’ things that I supposedly know about Gov. Blagojevich and Sen. Obama,” Rezko wrote in an undated letter released by the court this week. “I have never been party to any wrongdoing that involved the governor or the senator. I will never fabricate lies about anyone else for selfish purposes. I will take what comes my way, but I will never hurt innocent people.”

Randall Samborn, a spokesman for the U.S. attorney’s office in Chicago, wouldn't comment on Rezko's allegation.

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Obama’s campaign released a statement after the letter was made public, stressing that Obama hasn’t been accused of “any improper action or conduct involving Tony Rezko” and reiterating that he hasn’t been contacted for an interview or for any information about Rezko. “Nothing in this letter indicates anything to the contrary.”

Still, Republicans have seized on the relationship between the two, which includes a real estate deal that enlarged the Obama’s Chicago homestead, to question Obama’s judgment.

Blagojevich — who, like Obama, staked out an image as a political reformer — repeatedly found himself the subject of plenty of unwanted scrutiny during the trial, which prosecutors used to detail a complicated pay-to-play scheme involving state government boards and commissions.

Chicago media have reported that Rezko is being pressured to cooperate with ongoing investigations of Blagojevich’s campaign and administration.

But Rezko’s lawyer, Joe Duffy, told the Chicago Tribune that he did not know of any investigation into Obama’s dealings with Rezko.

“I'm not aware of any impropriety related to Rezko and Obama,” Duffy told the Tribune. "At no point has the government ever asked me a single question about Obama or any wrongdoing involving Rezko and Obama."

It’s not uncommon for prosecutors to try to get defendants to implicate more powerful or wealthy officials, said Stan Brand, who has represented scores of public officials both in private practice and as legal counsel to the U.S. House.

But proving that “they’ve actually crossed an ethical line and asked this guy to embellish or shade the truth in a way that’s helpful to them in some other case,” Brand said, is “a very difficult case to make absent some express coercion, which is very rare.”

The letter, apparently written during the two-month trial, was actually a plea to allow Rezko to see his family.

But its allegations about pressure to implicate Obama will surely give ammunition to partisans on both sides.

For Democrats, Rezko’s claim dovetails with allegations that the Department of Justice targeted high-profile Democrats.

For Republicans, it will be used to suggest Obama’s relationship with Rezko drew warranted federal scrutiny.